Diabetic retinopathy is a leading cause of vision loss in developed countries. Progression to sight threatening stages of diabetic retinopathy, including diabetic macular edema (DME), is usually associated with worsening of underlying retinal vascular dysfunction and disease, including an increase in the number and severity of retinal hemorrhages. Recent clinical advances have demonstrated that intravitreal injection of anti-VEGF directed therapies are effective in improving visual acuity and resolving macular edema in DME, and the FDA has recently approved intravitreal injection of ranibizumab for DME. While this treatment is highly effective for many patients with DME, several large clinical studies have revealed that about 40 to 50% of study subjects appear refractive or do not fully respond in the improvement of visual acuity and the resolution of retinal thickening to anti-VEGF directed therapies. These clinical findings suggest that VEGF-independent mechanisms might also contribute to retinal edema in a large number of DME patients. In recent reports and additional preliminary data, we show that plasma kallikrein (Pkal), factor FXII, and kininogen are increased in the vitreous of patients with DME. We also show that Pkal levels in the vitreous do not correlate with VEGF levels, revealing a component of molecular heterogeneity among DME patients. Using both knockout and pharmacological approaches, we show that Pkal contributes to retinal vascular hyperpermeability and retinal thickening in rodents and we have begun to characterize both bradykinin-dependent and -independent mechanisms that contribute to this response. Our studies have revealed that diabetes increases the effects of Pkal on vascular dysfunction and retinal edema (Clermont et al Diabetes 2011, Liu et al Nature Medicine 2011). We have also recently shown that autologous blood introduced into the vitreous increases retinal inflammation and retinal vascular permeability (Liu et al IOVS 2013) and the kallikrein system is as potent as VEGF in inducing retinal thickening. In addition we have used proteomics to characterize bradykinin-induced retinal edema and have identified a set of plasma proteins that are robustly increased in retinal edema coupled with striking decreases in a subset of retinal intermediate filament proteins. This grant will identify the PK- induced mechanisms that cause retinal vascular hyperpermeability and retinal thickening, and examine the hypothesis that the Pkal-bradykinin system is a VEGF-independent mediator of retinal dysfunction that contributes to DME. The specific aims are 1) to characterize and compare the bradykinin receptor-dependent and independent effects of Pkal on retinal edema in diabetic mice, 2) to compare Pkal and VEGF-induced retinal edema and investigate interactions between these pathways, and 3) characterize the roles of nitric oxide synthase and calpain-mediated intermediate filament degradation/remodeling in retina edema. Further identification of the mechanism of action and regulation of the plasma kallikrein system in the retina will be helpful in evaluating potential clinical opportunities for targeting the Pkal system for the treatment of DME.